In the mining industry, gold from gold ore is generally extracted by milling the gold ore sufficiently to allow separation of the gold and then utilizing various recovery processes such as amalgamation, cyanidation, gravity concentration, flotation and roasting or a combination of any of these. A common process used in the art is that of cyanidation. In the process of cyanidation the ore (or tailings) is leached with an alkaline cyanide solution, usually a solution of sodium cyanide (0.02-0.3%) or an equivalent of calcium cyanide together with a quantity of alkali such as lime or caustic soda in the presence of air (oxygen) or hydrogen peroxide. It is generally believed that the dissolution of gold by cyanidation occurs by the following equation: EQU 2Au+4NaCN+1/2O.sub.2 +H.sub.2 O.fwdarw.2Na(Au(CN).sub.2)+2Na(OH)
Air is a somewhat inefficient source of oxygen. It has been suggested that use of pure oxygen would optimize the process and this has been found to improve the recovery of gold as the rate of dissolution of gold is directly proportional to the oxygen content of the gas used for aeration.
Following cyanidation gold is then recovered by known means such as treating the solution with zinc dust or aluminium (Merrill-Crowe process); or removing gold from solution with activated carbon and then stripping the gold from the carbon with alcoholic caustic and reactivating the carbon by controlled roasting. The latter process is particularly suitable in the leaching or dilute ores by the carbon-in-pulp (CIP)/carbon-in-leach (CIL) process.
Generally, recovery of about 90% of the contained ore can be obtained with 10% of the gold being retained in the ore (refractory ore). In some ores the presence of base metals and silver will lead to an intolerably high consumption of cyanide. Economically, a cyanide usage of 1 to 2.5 kg NaCN per metric ton of ore is desired. A further disadvantage is that some gold containing ores may contain organic matter, ferrous compounds, arsenopyrite and/or pyrrhotite which may represent the major part of the oxygen demand.